Strobing numbers show saccadic vision

This week’s New Scientist has a brief letter which describes an elegant demonstration of visual processing during eye movements.

When you move your eyes (known as a saccade), visual input is suppressed, so less information is processed by the brain during the move.

This can be easily demonstrated, as described in one of the hacks in the Mind Hacks book (pdf).

An earlier article in New Scientist suggested that visual perception shuts down completely during the move, and someone wrote in with an elegant demonstration to show that this isn’t the case.

It is not strictly true that your visual perception mechanism shuts down completely during a rapid eye movement (22 September, p 34). This can easily be confirmed by flicking your eyes across a digital clock running off an alternating-current power supply, whose figures are luminous and flash at 100 or 120 hertz. You may see a line of images of the numbers, spaced out in proportion to the speed of your eye movement.

The same applies to a TV image: flicking your eyes to the right or left produces a succession of lozenge-shaped images, whereas flicking up or down results in a series of images respectively drawn out or squashed. If you flick your eyes down fast enough you can reduce the picture to a single bar, and if you flick your head down at the same time you can even manage to invert the picture, though you have to be very quick. Do not attempt this, however, if anyone is watching you.